Today, hand held spray dispensers for hair sprays are typically either of the manually-actuated spray pump type or the aerosol spray type. Aerosol spray dispensers utilize a liquefied propellant that "flashes off", to create an ultra fine spray. These ultra fine sprays have mean droplet diameters or mean particle sizes on the order of about 40 microns. When the propellant "flashes off", the phase change causes the liquid to disintegrate into ligaments and droplets. Although the small mean droplet diameter of ultra fine sprays produced by aerosols tends to leave a desirable dry feel on the hair, aerosols continue to be the subject of environmental debates. Therefore, many consumers prefer to use manually-actuated spray pump dispensers.
Manually-actuated spray pump dispensers or finger pumps rely on the consumer to generate a hydraulic pressure in the pumping engine in order to dispense the fluid. Most pumping engines typically use a standard piston and cylinder arrangement in order to generate this hydraulic pressure. Thus, when the consumer applies an actuation force by pushing downward on the piston, the hydraulic pressure of the fluid in the cylinder is increased. For example, in a pressure swirl nozzle type spray pump dispenser, the hydraulic pressure created in the pumping engine forces fluid into a pressure swirl nozzle that imparts a rotational motion to the fluid. The fluid spins inside of the nozzle and forms a thin conical sheet which exits into the atmosphere and breaks up into ligaments and droplets.
One fluid of current interest that requires the generation of a high hydraulic pressure in order to be properly dispensed by a manually-actuated spray pump dispenser is hair spray. Most manually-actuated spray pump dispensers have been unable to produce sprays having a mean droplet diameter of less than about 55 microns for many of the hair spray fluids currently on the market. These larger mean particle sizes, i.e. greater than about 55 microns, produced by conventional manual spray pumps result in sprays that consumers refer to as "wet". The wet and sticky feel of such sprays is due to the longer drying time required to dry the larger-sized particles. Several methods have been proposed for reducing the mean particle size produced by conventional manual spray pumps, for example, one of which is to increase the amount of hydraulic pressure created within the spray pump. Typically, most conventional spray pumps operate at a hydraulic pressure of about 90 psig. Research has indicated that when the hydraulic pressure in these conventional spray pumps is increased upward to levels near about 200 psig, mean droplet diameters of about 40 microns or less are achievable when used with a swirl type nozzle.
A method of developing a high hydraulic pressure of about 200 psig involves the use of a preloaded or precompression type outlet valve that will not open until the desired high hydraulic pressure (that is 200 psig) is reached. In order to reach these high hydraulic pressures, typically the stiffness of a precompression spring is increased. A stiffer precompression spring will prevent opening of the outlet valve until the desired high hydraulic pressure criteria is met. However, with this type of an outlet valve arrangement, the actuation force to be applied on the plunger that is required to dispense fluid from such a conventional spray pump can range from about 10 lbf to about 20 lbf. An actuation forces in this range is far too excessive for most ordinary consumers. Such an actuation force at this level can quickly fatigue the finger and hand of even the most physically adept person, let alone the typical users of most finger pumps.
Thus, a need exists for a manually-actuated spray pump that is capable of delivering substantially higher hydraulic pressures than conventional spray pumps without a corresponding increase in the actuation force which can be used to provide an ultra fine spray from a non-aerosol dispenser.